... there has been a reduction in the average brain size in South Africa during the last 10,000 years, and there have been parallel reductions in Europe and China -- pretty much everywhere we have decent samples of skeletons, it looks like brains have been shrinking. ... it is probably a matter of energetic efficiency (brains are expensive), developmental time (brains take a long time to mature) and diet (brains require high protein and fat consumption, less and less available to Holocene populations).

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It's a standard prediction of Darwinian considerations - childbirth mortality is a severe selection pressure, so is child mortality over a long maturation outside the womb, and if some gain in efficiency or architecture would preserve the intelligence advantage in a smaller package its evolutionary advantage would be very likely decisive.

Just in general, phylogenetically and looking at organisms comparatively, organisms with larger brains do tend to be smarter.

But it isn't absolutely linear.

I've always been struck by the birds, with tiny little heads but very alert and capable of very complex behavior.

And with regards to humans, Hawks might be on to something when he points out that brains are expensive in energy and nutrients.

So not only would there be selective value in growing the brain, since that would presumably correspond to smarter organisms, there would also be advantage in shrinking the brains as long as that didn't impact intelligence too dramatically. So combining the two contrary imperatives and we might have selective pressures towards an efficiency that can do more with less, so to speak. Some biological equivalent of algorithmic efficiency.

Just in general, phylogenetically and looking at organisms comparatively, organisms with larger brains do tend to be smarter.

But it isn't absolutely linear.

I've always been struck by the birds, with tiny little heads but very alert and capable of very complex behavior.

And with regards to humans, Hawks might be on to something when he points out that brains are expensive in energy and nutrients.

So not only would there be selective value in growing the brain, since that would presumably correspond to smarter organisms, there would also be advantage in shrinking the brains as long as that didn't impact intelligence too dramatically. So combining the two contrary imperatives and we might have selective pressures towards an efficiency that can do more with less, so to speak. Some biological equivalent of algorithmic efficiency.

Click to expand...

Is it possible that the overall size of the brain has shrunk, but that the surface area has actually increased, i.e. more folds allowing for greater density of neurons.
That would accomplish both desired effects of resource and energy management.

I believe this theory as to why brain size reduction has been happening is quite mainstream among biologists. The reduction has been observed continuously since the early 1900’s — the only real conjecture is the question why? I would argue that it’s a variety of factors, specifically in recent history the advancements of agriculture perhaps weren’t as intellectually challenging or maybe the diversity of their diets not as pervasive. Like most patterns of evolution it seems hard to pinpoint exactly the how and the why of it all, but energy consumption and compartmentalization seem most likely.

OK
Back at it.
So far, there seems to be broad agreement that most of us are experiencing a multi-generational shift to smaller brains.
One could go so far as to say that this started with heidelbergensis--with rapid expanse of the cranial cavity for millennia, topping out at 1800 cc then shrinking through neanderthalensis to us. But that is a different subject than the one for this thread.

From an average(from extent fossils) of 1650 cc for cromagnon to a modern average of well under 1450 we have lost more than 12% of cranial capacity. Average embraces quit a wide range with a lower limit of 1200cc.

[there is no proven 1:1 ratio of brain size to intelligence----the inverse is also true]

Ok then:
The question of causality comes into play.
Many speculations are to be found. What seems most likely(to me) is a 2 pronged hypothesis.
1) We ain't hunter gatherers no more---we have a different diet and different needs. We have a more stable food source and do not have to live by our wits.
Diet---we eat more grains and less wild stuff. We eat less protean and more carbs. The grains we eat lead to more inflamation
which means that we need to invest more in our immune system to fight inflammation. So being as both the brain and immune system are energy hungry, we rob one to feed the other. Wild fruits and berries and grains have more anti-oxidants per calory than do domesticated crops. So, we need to make up the difference with our immune systems.
So we sacrifice feeding a larger brain to accommodate our lower protean lower omega 3 fatty acids in favor of feeding our immune systems.

Conclusion:
We are experiencing a result of exceeding the natural carrying capacity of the land.
We have chosen a larger and more stable population in exchange for the evolution that brought us to the genius of cromagnon.